Use of genetically engineered mice permits investigators to study the development and function of the nervous system in vivo and associate gene function with disease. To generate these mice, investigators must first design and construct complex knock-in, knock-out, reporter gene or transgenic constructs. This represents a major obstacle for many labs, particularly because these large DNA constructs are difficult to engineer using classic molecular cloning techniques. To overcome this obstacle, we established the BAG Engineering Technology Core (Core 3) at the UNC Neuroscience Center. BAG Engineering (also called Recombineering) is based upon a highly efficient phage-derived . Co// homologous recombination system to efficiently and precisely engineer large DMA constructs. Our core specifically uses the Lambda-RED cloning system (Copeland et al., 2001). In the 3 and !4 years since the core was established, we used the Lambda- RED system to generate a diverse array of DNA constructs for NINDS-funded investigators, including gene targeting constructs (knock-in / knock-out and Cre / LoxP based) and BAG transgenes (to epitope tag proteins in vivo, to direct the expression of GFP and CRE proteins in a tissue specific manner). The NIH-funded GENSAT project highlights the utility of using BAG transgenic mice to study nervous system function (Gong et al., 2003; Heintz, 2004).